As an apparatus of such a type, there has been widely known an apparatus for controlling an air-fuel ratio of an internal combustion engine on the basis of the outputs from an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor provided in an exhaust passage (refer to, for example, Japanese Patent Application Laid-Open (kokai) Nos. Hei 6-317204, 2003-314334, 2004-183585, 2005-273524, etc.). The upstream air-fuel ratio sensor is disposed upstream of an exhaust purification catalyst for purifying an exhaust gas discharged from cylinders of the engine (the furthest upstream exhaust purification catalyst when two or more exhaust purification catalysts are provided) with respect to the flow direction of the exhaust gas. The downstream air-fuel ratio sensor is disposed downstream of the exhaust purification catalyst with respect to the flow direction of the exhaust gas.
As the above-described downstream air-fuel ratio sensor of such an apparatus, there is widely used a so-called oxygen sensor (also referred to as an O2 sensor) which exhibits a stepwise response in the vicinity of the stoichiometric air-fuel ratio (Z-characteristic: a characteristic in which the output of the sensor changes stepwise in such a manner that it changes suddenly when the air-fuel ratio changes between the rich and lean sides with respect to the stoichiometric air-fuel ratio). Meanwhile, as the above-described upstream air-fuel ratio sensor, there is widely used the-above described oxygen sensor or a so-called A/F sensor (also referred to as a linear O2 sensor) whose output changes in proportion to the air-fuel ratio.
In such an apparatus, the fuel injection amount is feedback-controlled on the basis of the output signal from the upstream air-fuel ratio sensor such that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes equal to (coincides with) a target air-fuel ratio (hereinafter, this control will be referred to as a “main feedback control”). In addition to the main feedback control, the output signal from the downstream air-fuel ratio sensor is used for a control for feeding back to the fuel injection amount (hereinafter, this control will be referred to as a “sub-feedback control”).
Specifically, in the main feedback control, a feedback correction amount is calculated in accordance with a difference between the air-fuel ratio of the exhaust gas (exhaust air-fuel ratio) corresponding to the output from the upstream air-fuel ratio sensor and the target air-fuel ratio. Meanwhile, in the sub-feedback control, a sub-feedback amount (sub-feedback correction amount) is calculated on the basis of the output signal from the downstream air-fuel ratio sensor. By means of feeding the sub-feedback amount back to the main feedback control, the difference between the exhaust air-fuel ratio corresponding to the output from the upstream air-fuel ratio sensor and the target air-fuel ratio is compensated.
Incidentally, as the above-described exhaust purification catalyst, there is widely used a three-way catalyst which can simultaneously remove from exhaust gas unburned substances, such as carbon monoxide (CO) and hydrocarbon (HC), and nitrogen oxide (NOx). Such a three-way catalyst has a function referred to as an oxygen occlusion function or an oxygen storage function. With this function, (1) in a case where the air-fuel ratio of the air-fuel mixture is on the lean side, nitrogen oxide contained in the exhaust gas is reduced through removal of oxygen therefrom and the removed oxygen is occluded (stored) in the three-way catalyst; and (2) in a case where the air-fuel ratio of the air-fuel mixture is on the rich side, the stored oxygen is released so as to oxidize the unburned substances contained in the exhaust gas.
The above-described oxygen storage function (i.e., an ability to purify exhaust gas) of such a three-way catalyst can be maintained at a high level by activating a catalytic material (noble metal) through repetitive storage and release of oxygen. In view of the above, there is known a technology (perturbation control) to forcibly oscillate/fluctuate the air-fuel ratio of the exhaust gas (i.e., the air-fuel ratio of the air-fuel mixture) so as to cause the three-way catalyst to store and release oxygen repeatedly in such an apparatus (refer to, for example, Japanese Patent Application Laid-Open (kokai) Nos. Hei 8-189399, 2001-152913, 2005-76496, 2007-239698, 2007-56755, 2009-2170, etc.).